Eccentric exercise has been demonstrated to provide a powerful strengthening stimulus to skeletal muscle. This type of exercise can benefit both the healthy athlete and injured patient because in both cases the result of the activity is increased muscle strength. Such eccentric exercise paradigms are commonly-termed "plyometrics" or "negatives." The objectives of this proposal are to determine the biomechanical mechanisms responsible for skeletal muscle injury after intense eccentric exercise. Our initial studies probe the biomechanical basis of muscle cellular injury using a mouse n vitro model. By measuring muscle sarcomere length during eccentric contractions which are induced under conditions of varying stress, strain, starting length and activation frequency, we will determine which biomechanical factors are the primary cause of injury. We will also measure the passive sarcomere length-tension relationship in these muscles after injury to quantify the functional result of the cytoskeletal disruption which has been reported. Using the rat motor unit model, we will test the hypothesis that selective injury to fast-glycolytic (FG) muscle fibers that occurs after eccentric contraction is due to the disproportionately short FG fibers within the muscle. This will be accomplished using physiological motor unit testing combined with complete three-dimensional reconstruction of muscle fibers belonging to the tested motor unit. This would provide a relatively simple geometrical explanation for the selective FG fiber injury which we and others have reported and provides the most rigorous test of fiber type specific injury performed to date. Finally, we will test the hypothesis that muscle injury is required for strengthening. This concept forms the basis for the term "no pain, no gain" but is based on very indirect evidence. Muscle injury and strengthening activity will be quantified by probing for various myosin heavy chain (MHC) transcripts after eccentric exercise. Based on the type and quantity of each MHC transcript expressed, we can determine the relationship (if any) between cellular injury and strengthening. Taken together, these studies will provide information which can be used to minimize muscle injury and to optimize strengthening protocols used during physical rehabilitation after injury or disuse.